known and unknown causes of variation."[342] He admits,
indeed, the effects of use and disuse to have been important, but how
important we have no means of knowing; he also attributes considerable
effect to the action of changed conditions of life--but how considerable
again we know not; nevertheless, he sees no great principle underlying
the variations generally, and tending to make them appear for a length
of time together in any definite direction advantageous to the creature
itself, but either expressly, as at times, or by implication, as
throughout his works, ascribes them to accident or chance.

In other words, he admits his ignorance concerning them, and dwells only
on the accumulation of variations the appearance of which for any length
of time in any given direction he leaves unaccounted for.

Lamarck, again, having established his principle that sense of need is
the main direct cause of variation, and having also established that the
variations thus engendered are inherited, so that divergences accumulate
and result in species and genera, is comparatively indifferent to
further details. His work is avowedly an outline. Nevertheless, we have
seen that he was quite alive to the effects of the geometrical ratio of
increase, and of the struggle for existence which thence inevitably
follows.

Mr. Darwin, on the other hand, comparatively indifferent to, or at any
rate silent concerning the causes of those variations which appeared so
all-important to Lamarck, inasmuch as they are the raindrops which unite
to form the full stream of modification, goes into very full detail upon
natural selection, or the survival of the fittest, and maintains it to
have been "the most important but not the exclusive means of
modification."[343]

It will be readily seen that, according to Lamarck, the variations which
when accumulated amount to specific and generic differences, will have
been due to causes which have been mainly of the same kind for long
periods together. Conditions of life change for the most part slowly,
steadily, and in a set direction; as in the direction of steady, gradual
increase or decrease of cold or moisture; of the steady, gradual
increase of such and such an enemy, or decrease of such and such a kind
of food; of the gradual upheaval or submergence of such and such a
continent, and consequent drying up or encroachment of such and such a
sea, and so forth. The thoughts of the creature varying will thus ha